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Having dropped off their period costumes at the dry cleaners, it’s back to Bristol for the Doctor and Bill this Saturday.

The upcoming fourth episode is titled (and we can’t believe they didn’t think of this before) Knock Knock.

Written by Mike Bartlett (Doctor Foster), this one is set to chill you to the bone – presumably before gnawing the flesh right off it.

Bill gets more than she bargained for when she rents a house with some friends, only to find they’re sharing it with some rather creepy termites.

Here’s what we know.

What happened last week?

Episode 3, Thin Ice, was a foggy caper in Regency London.

It saw the Doctor and Bill investigate a spate of mysterious disappearances – all connected to a hungry serpent living under the Thames that’s in the control of the cruel Lord Sutcliffe (Nicholas Burns).

Bill discovered a different side to her mentor as he fluctuated between apparent indifference over the death of a street urchin and righteous anger when he punched out Sutcliffe for his racist attitudes.

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At the end, Nardole had a half-conversation with whatever it is that’s secured in the vault – and it seemed rather anxious to get out.

Who’s in episode 4, and what’s it like?

(Picture: BBC/BBC Worldwide/Jon Hall)

This week’s special guest star is none other than Poirot himself, David Suchet, playing a character known only as the Landlord.

‘You signed the contract,’ he mutters in the trailer, ‘now it’s time to pay’.

We assume this refers to something rather more serious than a tenancy deposit.

There’s a mystery surrounding the tower at the building’s heart, which Bill and her friends are forbidden to enter, and something very nasty coming out of the walls.

To add to the general spookiness, it was filmed at Fields House in Newport, which was also the location for Wester Drumlins, the Weeping Angel-infested house that Sally Sparrow explored in Blink (2007).

There are no Angels in sight this week, but there is a rather creepy-looking wooden thing that looks like the product of a marriage between Pinocchio and Medusa.

When is Doctor Who on and where can I watch it?

(Picture: BBC / Simon Ridgway)

Knock Knock will be broadcast on Saturday, May 6, at 7.20pm on BBC One.

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Anything else?

To up the creep factor even further, there’s going to be a special binaural edition of this week’s episode, featuring ‘a 3D surround sound effect for anyone wearing headphones’.

It will be released on iPlayer just after transmission – but you might want to leave the lights on.